1. Field of the Invention
The present invention relates to a process for measuring the hardness of ductile iron castings using Brinell measuring apparatus. In particular, this invention relates to an automatic process for such hardness measurement whereby an accept/reject decision can be made automatically and accurately.
2. Prior Art
The Brinell Hardness test is an indentation hardness test using calibrated machines to force a hard ball, under specified conditions, into the surface of the material under test and to measure the diameter of the resulting impression after the removal of the load. From this measurement, a Brinell Hardness number is developed which is related to the applied load and to the surface area of the permanent impression made by a ball indenter computed from a mathematical equation. The mathematical equation, which is not important for purposes of this application, includes variables such as applied load, diameter of the ball and the mean diameter of the impression.
Historically, the Brinell method of hardness determination has been used for the purpose of verification or reference when a high degree of accuracy in metal castings is required, and alternately in some routine tests where somewhat lower degree of accuracy is permissible. There has, however, been a substantial amount of difficulty in using the Brinell Hardness system and apparatus where a high degree of accuracy is routinely required. As a specific example, an automobile casting, and especially those castings which are used in especially sensitive parts such as brake and steering systems, need to be tested constantly for hardness. Although the Brinell system is an established and known system for such measurements, it has not been possible in the past to automate such a hardness test to the point of routine use.
By practice of the process of this invention, it is possible to routinely use Brinell measurement apparatus to automatically measure the hardness of a metal casting and to create a permanent record with regard to hardness from which an appropriate accept/reject decision can be made as to the ultimate casting. The permanent record can also be retained for future verification of casting standards and specifications.
One of the areas of novelty in this invention is the surface preparation step which involves the use of what will be referred to hereinafter as a contrast enhancement material. The precise function of this material and the significance of this surface preparation step will become apparent after a further description of the Brinell reading system itself.